Technology update

Jul 19, 2016

Graphene composite enables metre-sized flexible displays

The high demand for flexible, transparent conductors had posed a seemingly promising avenue for capitalizing on the substantial research already invested in graphene. However, the conductivity of pristine single-layer graphene is poor compared with its brittle counterpart indium tin oxide (ITO). Researchers in Exeter devised a graphene composite – GraphExeter – that is highly conducting, and now they demonstrate the first prototype of the material in an electroluminescent device, showing its feasibility for luminescent textiles and flexible displays.

"We live in the future," says Saverio Russo, who heads the team of researchers working on GraphExeter. He describes how the discovery of the material’s superlative properties consigned certain limitations of current technology to history, in theory. The working prototypes using the material - like this recently reported display – have now given the optimistic proposals for GraphExeter sound footing in realizable devices.

The high conductivity of GraphExeter does more for flexible displays than just brighter screens, Russo tells nanotechweb.org. Large screens using existing materials fade to a darker region in the middle due to the sheet resistance. This places a practical limitation in the range of square centimetres on how large the screen can be. The extremely low sheet resistance of GraphExeter - less than 8Ω/□ compared with 1000Ω/□ for pristine single layer graphene and 850Ω/□ for the more commonly used PEDOT/PSS – means that screens several square metres in size are now feasible.

What is GraphExeter?

Russo describes the material as “like a lasagne”, with FeCl3 molecules sandwiched between the single layers of graphene. These molecules donate additional charge carriers, which are responsible for the huge decrease in sheet resistance compared with pristine graphene.

The researchers discovered the material while exploring a different potential application of graphene altogether. “We were interested in whether it could be used to make magnetic memory in graphene,” says Russo. Iron chloride molecules are ferroelectric, so incorporating them seemed a promising approach. Although this is still an open avenue of research within the group at Exeter, it soon became apparent that the material had other very readily exploitable attributes.

As well as its high conductivity, GraphExeter has 85% transparency and is stable up to 100% humidity and 150 °C. It is also easy to fabricate with a large surface area using chemical-vapour-deposition (CVD)-grown graphene, and resists degradation for more 1000 bending cycles to a radius of curvature of 3 mm.

GraphExeter is also free from many of the undesirable features of the alternatives – PEDOT/PSS has a blue tinge and is hygroscopic, so that at humidity any higher than 40% it begins to absorb water and the conductivity deteriorates. The nanowire meshes that have also been explored are prone to optical haze, electromigration and poor electrical stability. “It is 50% brighter than commercial graphene and 30% more efficient than commercially available transparent electrodes such as PEDOT:PSS,” adds fellow team member Elías Torres Alonso.

What are its uses?

Such a high-performing, flexible display could be useful for foldable displays, lighting systems with expandable screens for smart phones, wallpaper lights and optoelectronic textiles. In particular, Russo suggests how useful the material could be as a biocompatible light source for medical treatments such as clothing for premature babies. With GraphExeter, a UV source directed towards the skin can be integrated into the fabric, so the eyes are safe from harmful radiation without the need for goggles.

The researchers had previously demonstrated the use of GraphExeter in a pressure sensor that can be worn on the wrist to monitor heartbeat. The hope is that these working prototypes will emphasize the value of the material for facilitating new technologies, and thereby attract further corporate or government investment to take the commercialization further.